Motor controllers for AC-motors etc. need to know whether an earth-fault (common-mode fault) exists during operation. This type of fault should be distinguished from differential-mode faults such as for example over currents caused by a blocked rotor of the motor.
Those skilled in the art will acknowledge the following priority of excessive current in a motor controller.                a. Short-circuit currents in the range of the saturation level of the switching elements should cause a permanent shutdown to be initiated within microseconds regardless of whether the low-impedance problem is of a common- or differential-mode nature.        b. Over currents caused by common-mode faults with current-limiting impedance in the earth loop should be limited to an upper level for a given time period in the range of milliseconds before a permanent shutdown is initiated.        c. Over currents caused by load-related differential-mode faults should be limited to an upper level for a given time period in the range of seconds before a permanent shutdown is initiated.        
Motor controllers with a short-to-earth on an output phase are likely to be categorized as exhibiting an item b. problem, when the rectifier-side employs inductance for line-current harmonics limitation in accordance with IEC1000-3-2 or IEC1000-3-12. The difference between item b. and c. is that common-mode faults will stress the rectifier part with high-frequency currents, which differential-mode faults do not. The rectifier stress may lead to catastrophic failures if the duration exceeds the millisecond range.
The VLT5000 produced by Danfoss Drives NS since 1995 employs 3 current transducers on the output phases. By summing the current transducer signals an earth-fault signal is generated. Hence, this principle is always able to differ between differential and common-mode faults. The drawback of this solution is cost-related.
The VLT2800 produced by Danfoss Drives NS since 1998 employs a shunt in the lower DC-link bus to sense differential-mode currents, and a common-mode summing current transformer in the rectifier part to sense earth currents. Hence, this principle is able to differ between differential and common-mode faults. A similar approach is suggested in U.S. Pat. No. 5,687,049 where the summing transformer is placed in the inverter stage of the motor controller. Though this solution exhibit lower costs than the VLT5000 solution, both solutions will result in a problematic PCB layout due to more current sensing elements in the DC-link.
U.S. Pat. No. 5,687,049 proposes a solution with a high and low-side current sensing element in the inverter part of the DC-link bus. Summing the 2 sensing signals (where at least one needs to employ galvanic/functional isolation) gives an earth-current signal similar to the VLT5000 solution. Hence, this solution is able to differ between item b. and c. However, the practical PCB layout issue is a drawback.
The IAS'96 conference paper “Single Current Sensor Technique in the DC-link of Three-phase PWM-VS Inverters A Review and the Ultimate Solution” and U.S. Pat. No. 5,687,049 report a solution with a current transducer having both the positive and negative DC-link bus wired through the transducer with an unequal number of turns. This reduces the count of current sensing elements to unity and is reported as the “ultimate solution” for protecting a motor controller (will differ between item b. and c.). However, those skilled in the art will acknowledge that this kind of multiple turns in a current transducer may compromise an optimum coupling and give excessive leakage inductance in the inverter side of the DC-link. Also, the PCB layout is problematic. Furthermore, design and automatic assembly become difficult with multiple turns having different voltage potentials in a modern small-sized current transducer.
Hence, the present invention is preferably used along with the following hardware combinations considered to be best-suited for a modern, low-cost and robust motor controller.                1. Motor controller with an inverter stage employing a shunt in series with each of the low-side switching elements and desaturation protection of the high-side switching elements.        2. Motor controller with a shunt in the low-side DC-link bus and de-saturation protection of the high-side switching elements.        3. Motor controller with a current transducer in the low- or high-side DC-link bus and de-saturation protection of the high- or low-side switching elements.        
Items 1 and 2 assume that the control circuitry for the motor controller is referenced to the low-side DC-link bus. Item 3 assumes that the control circuitry is galvanically isolated from the power stage. The current sensing element(s) gives a feedback to the control circuitry in the motor controller. The de-saturation protection is used to protect the switching elements in the opposite side of where the current sensing element(s) is placed. The de-saturation protection may be with or without galvanically/functionally isolated feedback to the control circuitry as described in U.S. Pat. No. 5,687,049.
De-saturation protection without feedback is patented in U.S. Pat. No. 5,687,049 meaning that the switching elements with de-saturation protection operates on a self-protective, cycle-by-cycle basis until the central control circuitry shuts down the inverter stage in response to a fault signal from the current sensing element. De-saturation protection with feedback is well known and offered by many gate drive vendors at least since the early 1990th —for example the IXYS driver chipset IXPD4410 and IXPD4411.
The preferred hardware configurations will not be able to distinguish between a common-mode and differential-mode fault condition as the other solutions. Intelligent sampling of the DC-link current sensing element(s) is required. The above IAS'96 paper teaches that the earth-fault current may be sampled during the zero-voltage vectors 000 or 111. However, the method suggested in IAS'96 does not offer the possibility that the phase connected to the earth-fault may be identified.
Hence, the information is available on a switching-period basis as long as a zero-voltage vector is available. EP 0 490 388 discloses a principle of receiving an over-current fault signal, a first action is to generate a signal from the PWM sequence to determine whether the fault occurred during a zero-voltage vector or an active vector. This will differ between item b. and c. above. However, the patent does not consider the problem that the zero-voltage vector may not exist generally in all operating points.
An industrial standard for PWM generation is the space-vector-modulation reported in the PESC'90 conference paper “Stator Flux Oriented Asynchronous Vector Modulation for AC-Drives” (referred to as SFAVM below) along with all the variants of SFAVM generated by varying the zero-voltage-vector distribution. The goal of these PWM strategies is to obtain optimised motor performance in terms of torque and current ripple, losses, acoustic noise and voltage-transfer ratio from input to output. It is well known that SFAVM uses the zero-voltage vectors in every switching period at low output voltages. However, at high output voltage the use of the zero-voltage vectors is minimized. In some switching cycles the zero-voltage vectors may not be used, especially in the over-modulation range. And in some cycles the zero-voltage vectors may be used for a short time only, meaning that a proper measurement of an earth current during a zero-voltage vector becomes practically impossible. The problem becomes even worse as the switching frequency is increased.